The CHARA Array is a six-element, optical/NIR interferometer, which currently has the largest operational baselines in the world. The Array is operated by Georgia State University and is located at the Mount Wilson Observatory in California. The Array thrives thanks to members of the CHARA consortium that includes LESIA (Observatoire de Paris), Observatoire de la Cote dAzur, University of Michigan, Sydney University, Australian National University, and University of Exeter. Here we give a brief introduction to the Array infrastructure with a focus on a developing Adaptive Optics (AO) program, the new community access program funded by the NSF, and recent science results.

The near-infrared GRAVITY instrument has become a fully operational spectro-imager, while expanding its capability to support astrometry of the key Galactic Centre science. The mid-infrared MATISSE instrument has just arrived on Paranal and is starting its commissioning phase. NAOMI, the new adaptive optics for the Auxiliary Telescopes, is about to leave Europe for an installation in the fall of 2018. Meanwhile, the interferometer infrastructure has continuously improved in performance, in term of transmission and vibrations, when used with both the Unit Telescopes and Auxiliary Telescopes. These are the highlights of the last two years of the VLTI 2nd generation upgrade started in 2015.

The Large Binocular Telescope Interferometer is currently in routine operation for programs such as the Hunt for Observable Signatures of Terrestrial planetary Systems (HOSTS), and its associated exoplanet survey, LEECH. Additional modes of operation are in various stages of being demonstrated and planned, including imaging interferometry, integral field spectroscopy over the 1-5 micron range, and an update to the wavefront sensors for adaptive optics. We will present current results and instrumentation plans to continue exploiting the LBT as a unique facility for high spatial resolution, high sensitivity observations.

The Navy Precision Optical Interferometer (NPOI) is currently undergoing a fundamental renaissance in its functionality and capabilities. Operationally, its fast delay line (FDL) infrastructure is completing its upgrade from a VME/VxWorks foundation to a modern PC/RTLinux core. The Classic beam combiner is being upgraded with the New Classic FPGA-based backend, and the VISION beam combiner has been upgraded over this past summer with low-noise EMCCD cameras, resulting in substantial gains in sensitivity. Building on those infrastructure improvements, substantial upgrades are also in progress. Three 1-meter PlaneWave CDK1000 telescopes are being delivered to the site, along with their relocatable enclosure-transporters, and stations are being commissioned for those telescopes with baselines ranging from 8 meters to 432 meters. Baseline-wavelength bootstrapping will be implemented on the facility back-end with a near-infrared beam combiner under development. Collectively, these improvements mark substantial progress in taking the facility towards realizing its full intrinsic potential.

The Magdalena Ridge Observatory Interferometer (MROI) has been under development for almost two decades. Initial funding for the facility started before the year 2000 under the Army and then Navy, and continues today through the Air Force Research Laboratory. With a projected total cost of substantially less than $200M, it represents the least expensive way to produce sub-milliarcsecond optical/near-infrared images that the astronomical community could invest in during the modern era, as compared, for instance, to extremely large telescopes or space interferometers. The MROI, when completed, will be comprised of 10 x1.4m diameter telescopes distributed on a Y-shaped array such that it will have access to spatial scales ranging from about 40 milliarcseconds down to less than 0.5 milliarcseconds. While this type of resolution is not unprecedented in the astronomical community, the ability to track fringes on and produce images of complex targets approximately 5 magnitudes fainter than is done today represents a substantial step forward. All this will be accomplished using a variety of approaches detailed in several papers from our team over the years. Together, these two factors, multiple telescopes deployed over very long-baselines coupled with fainter limiting magnitudes, will allow MROI to conduct science on a wide range and statistically meaningful samples of targets. These include pulsating and rapidly rotating stars, mass-loss via accretion and mass-transfer in interacting systems, and the highly-active environments surrounding black holes at the centers of more than 100 external galaxies. This represents a subsample of what is sure to be a tremendous and serendipitous list of science cases as we move ahead into the era of new space telescopes and synoptic surveys. Additional investigations into imaging man-made objects will be undertaken, which are of particular interest to the defense and space-industry communities as more human endeavors are moved into the space environment.

In 2016 the first MROI telescope was delivered and deployed at Magdalena Ridge in the maintenance facility. Having undergone initial check-out and fitting the system with optics and a fast tip-tilt system, we eagerly anticipate installing the telescope enclosure in 2018. The telescope and enclosure will be integrated at the facility and moved to the center of the interferometric array by late summer of 2018 with a demonstration of the performance of an entire beamline from telescope to beam combiner table shortly thereafter. At this point, deploying two more telescopes and demonstrating fringe-tracking, bootstrapping and limiting magnitudes for the facility will prove the full promise of MROI. A complete status update of all subsystems follows in the paper, as well as discussions of potential collaborative initiatives.

After the first year of observations with the GRAVITY fringe tracker, we compute correlations between the optical path residuals and atmospheric and astronomical parameters. The median residuals of the optical path residuals are 180nm on the ATs and 270nm on the UTs. The residuals are uncorrelated with the target magnitudes for Kmag below 5.5 on ATs (9 on UTs). The correlation with the coherence time is however extremely clear, with a drop-off in fringe tracking performance below 3 ms.

MATISSE is the 2nd generation mid-infrared instrument designed to combine four VLTI telescopes in the L, M and N spectral bands. It’s commissioning in Paranal is in progress since March 2018 and should continue until the middle of 2019. Here we report, in June 2018, the commissioning plan, tools and the preliminary results of the first two commissioning runs in MATISSE that show that the instrument is already fully operational with a sensitivity well beyond its specification. The quality of the measurements, as they obtained by the current observing procedures and delivered by the current pipeline are already good enough for a broad range of science observations. However, our results remain quite preliminary and they will be quite substantially improved by the work in progress in instrument calibration, observing procedures optimization and data processing updates.

FIRST (Fibered Imager foR a Single Telescope) is a post-AO instrument module that enables high-contrast imaging and spectroscopy at sub-diffraction limited spatial scales. FIRST achieves this through a unique combination of sparse aperture masking, spatial filtering, pupil remapping Fizeau interferometry, and cross-dispersion in the visible. The telescope pupil is divided into sub-pupils using a honeycomb array of micro-electro-mechanical mirrors, and the light from each sub-pupil injected into a separate single mode fiber that provides spatial filtering. The fibers, which are pathlength-matched to within a few tens of micrometers, reformat the sub-apertures into a linear non-redundant array allowing for the extraction of fringes from each possible baseline as well as wavelength dispersion to create ~130 spectral channels for every baseline combination over the 600-900nm spectral range.
In this presentation, we will first report on the latest on-sky results obtained with FIRST. In its current design, the instrument was successfully integrated on the 3-m telescope at Lick Observatory and is now a module of the SCExAO extreme adaptive optics instrument on the 8-m Subaru Telescope. The latest on-sky results obtained from commissioning data show the detection of the stellar companion of the Alpha Equu binary system at an angular separation of 0.6 λ/D (11mas). Even at such a separation, the FIRST data delivers information on the companion spectrum, providing valuable constraints on the stellar parameters of the system such as the effective temperatures.
The second part of this presentation will focus on the ongoing instrument upgrades with the aim of increasing the instrument’s stability and sensitivity, thus improving the dynamic range. We initiated a comprehensive upgrade of FIRST’s interferometric components to a new series of photonic on-chip beam combiners and automated optoelectronic delay lines for rapid phasing of each sub-pupil. The new photonic beam combining chips split light from each sub-aperture and combines them to provide a simultaneous measurement of the fringes from every baseline. Another function of the new photonic chips is the inclusion of waveguides in crystalline electro-optic material (Lithium niobate) that enable on-chip active phase control of the light at high speeds (up to kHz). This type of photonic architecture has not been implemented previously for astronomical interferometry of this kind and could potentially provide FIRST with key advantages (see Martin et al., these proceedings).
While the beam-combiner output no longer requires non-redundancy, the fiber array that feeds the chip input still requires accurate pathlength-matching to achieve high fringe contrasts. The existing fibers were individually manufactured to ensure identical length. However, while this method was successful, it was not very flexible particularly if any photonic components are added that change the overall fiber length. Thus, another key FIRST upgrade is the use of actively controlled fiber delay lines capable of compensating for up to 100 mm of differential pathlength in each fiber, with sub-micron accuracy. This type of active pathlength control allows FIRST to not only correct for unwanted environmental phase delays, but also makes it entirely reconfigurable regardless of the back-end photonics used.

Since 1994, the Navy Precision Optical Interferometer (NPOI) has operated at visual wavelengths (450 to 850 nm). Its primary Classic backend is a pupil-plane combiner that disperses the light at a resolution R ≈ 50, uses avalanche photo-diodes as photon-counting detectors, and scans interference fringes by modulating the delay at 1 kHz. The newer NPOI image-plane combiner, VISION (Tennessee State University), which is similar to CHARA’s MIRC and is currently being upgraded, dispenses with delay modulation. We are now developing a third backend to expand into the near infrared. Its primary purpose will be to stabilize the NPOI for high-resolution observations by bootstrapping from the infrared to visual wavelengths.

The Magdalena Ridge Observatory Interferometer (MROI) is currently under construction in New Mexico at an altitude of 3.2 km. When completed it will consist of ten 1.4 m telescopes and will operate at wavelengths from 0.6 to 2.4 μm. Here we present the preliminary design of the Free-space Optical multi-apertUre combineR for IntERferometry (FOURIER), the first generation near infrared science beam combiner at the MROI which is currently under development. The combiner will operate in the J, H and K bands and combine three beams from the currently funded subset of three telescopes. The primary aim of the combiner is to achieve high sensitivity leading to its unique design.

The ICoNN (Infrared Coherencing Nearest Neighbors) fringe tracker system is the heart of the Magdalena Ridge Observatory Interferometer (MROI). It operates in the near-infrared at H or Ks in such a way that the light being used by the fringe tracker can phase up the interferometric array, but not steal photons from the scientific instruments of the interferometer system. It is capable of performing either in group delay tracking or fringe phase tracking modes, depending on the needs of the scientific observations. The spectrograph for the MROI beam combiner was originally designed for the Teledyne PICNIC array. Developments in detector technology have allowed for an alternative to the original choice of infrared array to finally become available – in particular, the SAPHIRA detector made by Selex. Very low read noise and very fast readout rates are significant reasons for adopting these new detectors, traits which also allow relaxation of some of the opto-mechanical requirements that were needed for the PICNIC chip to achieve marginal sensitivity. This paper will discuss the opto-mechanical advantages and challenges of using the SAPHIRA detector with the pre-existing hardware. In addition to a design for supporting the new detector, alignment of optical components and initial testing as a system are reported herein.

Piezoelectric actuators are essential elements for high-precision positioning in many astronomical applications (OPD modulation for fringe tracking systems, control of tip/tilt modes in Adaptive Optics....). This paper proposes a control method to take into account nonlinear dynamics of piezoelectric actuators such as hysteresis and creep. Some simulations demonstrate the effectiveness of the control approach and experimental results are obtained for the creep compensation.

NESSI and `Alopeke are two speckle imaging instruments for community use at the WIYN and Gemini-North telescopes. The two instruments were built at NASA ARC and include the capability for wide-field and traditional CCD imaging. Speckle interferometry utilizes extremely short exposures to produce interferograms from the turbulent atmosphere that are reconstructed into a diffraction-limited image, effectively giving space-based resolution from the ground. A primary role of these instruments is exoplanet validation for the Kepler, K2, TESS, and many RV programs. Contrast ratios of 6 or more magnitudes are easily obtained. The instrument uses two EMCCD cameras and two filter wheels to provide simultaneous dual-color observations in either narrowband or SDSS broadband filters to characterize detected companions. High resolution imaging enables the identification of blended binaries that contaminate many exoplanet detections, leading to incorrectly measured radii.

MATISSE (Multi AperTure mid-Infrared SpectroScopic Experiment) is the spectro-interferometer for the VLTI of the European Southern Observatory (ESO), operating in the L-, M- and N- spectral bands, and combining up to four beams from the unit or the auxiliary telescopes (UTs or ATs). MATISSE will offer new breakthroughs in the study of circumstellar environments by allowing the mapping of the material distribution, the gas and essentially the dust. The instrument consists in a warm optical system (WOP) accepting four beams from the VLTI and relaying them after a spectral splitting to cold optical benches (COB) located in two separate cryostats, one in L-M- band, and one in N-band. The test plan of the complete instrument has been conducted at the Observatoire de la Côte d’Azur in order to confirm the compliance of the performance with the high-level requirements. MATISSE has successfully passed the Preliminary Acceptance in Europe the 12th September 2017. Following this result, ESO gave approval for the instrument to be shipped to Paranal. The Alignment, Integration and Verification phase was conducted until end of February 2018, at the end of which first observations on sky have been performed to test the operations with the VLTI and to obtain first stellar light. The two first runs of the commissioning followed, respectively in March and in May 2018. It has the goal to optimize the MATISSE-VLTI communication, the acquisition procedures and the interface parameters. The observations were performed on bright L-M- and N- stars, with four ATs located on short baselines and UTs. The limit magnitudes will be deduced.

This paper reports on the performance of the instrument measured in laboratory (results of test plan in Nice and AIV in Paranal) in terms of spectral coverage, dispersion laws and spectral resolutions, and transfer function analysis: instrumental contrast, visibility accuracy, accuracy of the differential phase, of the closure-phase and of the differential visibility. It also provides results of the first tests on sky and the planning of the on-going commissioning.

Initial data for the current and ongoing experiment to measure and possibly predict the horizontal turbulent strength, C2N , of the atmosphere above the Magdalena Ridge Observatory Interferometer (MROI) is presented. C2N is a representation of the atmosphere’s ability to transport scalars and is measured using a set of Kipp and Zonen Large Aperature Scintillometers (LAS). LAS Calibration data as well as initial test data are presented and analyzed. Correlation techniques are used to determine the optimal method of C2N calculation from the first generation LAS. A 19-day test over the array site was conducted and analyzed using both Fourier and wavelet analysis and filtration. Frequency analysis showed few periodic features due to the quasi-periodic nature of the signal.

The Large Binocular Telescope Interferometer (LBTI) can perform Fizeau interferometry in the focal plane, which accesses spatial information out to the LBT's full 22.7-m edge-to-edge baseline. This mode has previously been used to obtain science data, but has been limited to observations where the optical path difference (OPD) between the two beams is not controlled, resulting in unstable fringes on the science detectors. To maximize the science return, we are endeavoring to stabilize the OPD and tip-tilt variations and make the LBTI Fizeau mode optimized and routine. Here we outline the optical configuration of LBTI's Fizeau mode and our strategy for commissioning this observing mode.

Since its first light at the Very Large Telescope Interferometer (VLTI), GRAVITY has reached new regimes in optical interferometry, in terms of accuracy as well as sensitivity.1 GRAVITY is routinely doing phase referenced interferometry of objects fainter than K > 17 mag, which makes for example the galactic center black hole Sagittarius A*2 detectable 90 % of the times. However from SNR calculations we are confident that even a sensitivity limit of K ~ 19 mag is possible. We therefore try to push the limits of GRAVITY by improving the observations as well as the calibration and the data reduction. This has further improved the sensitivity limit to K > 18 mag in the beginning of this year. Here we present some work we are currently doing in order to reach the best possible sensitivity.

Stable beam alignment of an optical interferometer is crucial for maintaining a usable signal-to-noise ratio during science measurements on faint astronomical targets. The Magdalena Ridge Observatory Interferometer will use an Automated Alignment System (AAS) that performs a start-of-night alignment procedure and subsequent alignment corrections in between observations, all without the need for human intervention. Its design has recently been updated in line with a revised error budget for MROI requiring that two axis drifts during science operations should not exceed 15 milliarcseconds in tilt, referred to the sky, nor 1% of the beam diameter in shear. For each beam line, the AAS provides two reference light beams, a pair of quad cells to monitor coarse alignment, and a tilt and shear detector for tracking fine drifts. The tilt and shear detector is a novel application of a Shack-Hartmann array that permits the simultaneous measurement tilt and shear well within requirements for MROI. Results of laboratory testing and simulations are presented here.

The New Adaptive Optics Module for Interferometry (NAOMI) is ready to be installed at the 1.8-metre Auxiliary Telescopes (ATs) at ESO Paranal. NAOMI will make the existing interferometer performance less dependent on the seeing conditions. Fed with higher and more stable Strehl, the fringe tracker will achieve the fringe stability necessary to reach the full performance of the second-generation instruments GRAVITY and MATISSE. All four ATs will be equipped between September and November 2018 with a Deformable mirror (ALPAO DM-241), a 4*4 Shack– Hartmann adaptive optics system operating in the visible and an RTC based on SPARTA Light. During the last 6 months thorough system test has been made in laboratory to demonstrate the Adaptive Optics and chopping capability of NAOMI.

The deployment of the Magdalena Ridge Observatory Interferometer has resumed in 2016. AMOS, in charge of the development of the unit telescopes, has completed the installation of the first telescope on the Ridge. The compactness of the system allows for a fast installation, as only the optics and their supports need to be transported in separate crates. The installation has been followed by the alignment procedure combining metrological and optical measurement techniques and aiming at optimizing the pupil stability and image quality. Finally, the performance of the telescope has been evaluated on the sky as part of the site acceptance.

The Unit Telescope (UT) for the Magdalena Ridge Observatory (MROI) is composed of four major hardware components: The Unit Telescope Mount (UTM), Enclosure, Optics and the Fast Tip Tilt System (FTTS). Integration of the UT started in 2016 when the UTM arrived and its Assembly, Integration and Verification activities began. Critical activities included: installation at the Maintenance Facility, integration and alignment of the Optics and Wave Front Sensor (WFS) and finally the complete optical alignment. End-to-end UTM Site Acceptance Tests (SAT) were performed. Subsequent activities included receiving and integrating the FTTS. With the arrival and assembly of the Enclosure, the last component of the UT was ready for integration on a dedicated concrete pier. Specialized equipment will be used for the final integration of the UT, and for transportation to its final location on the array where SAT for the UT will take place.

We propose using telescopes with an elongated-pupil to improve resolution as compared to circular-pupil tele- scopes. For the same aperture area and exposure time, an elongated-pupil telescope will have higher resolution on one axis than the maximum resolution of the circular-pupil on any axis. If the elongated-pupil is rotated around the optical axis and the same field is measured at different angles, a final image with a circular-symmetric point spread function can be reconstructed using proper image coaddition algorithms. We present simulations comparing a circular- and an elongated-pupil telescope and show the elongated-pupil telescope attains higher contrast at lower separation between a bright star and a faint companion. Further work, probing the advantages of elongated-pupil telescopes using simulations of imperfect optics, point spread function measurement errors, and atmospheric turbulence is currently underway.

The year 2017 marks the 150th anniversary of Fizeau's celebrated paper which first suggested the idea of harnessing the then-infant technology of interferometry in the service of astrophysical measurement. This took the form of a mask, to be placed over the entrance pupil of a telescope to create what we would now term a Fizeau Interferometer. The experiment was successfully performed at Marseilles a few years later. Despite its antiquity, this deceptively simple idea is still with us and thriving beyond reasonable expectations today: an aperture mask that would be recognisable to Fizeau will fly aboard the James Webb Space Telescope. This paper highlights remarkable results at the very finest angular scales still being delivered by this technique, and prospects for future innovations on Fizeau's idea that may well extend the winning streak well into the future. In particular, the idea of strict non-redundancy usually enforced in the layout of masking arrays is subject to scrutiny.

Sparse Aperture Masking (SAM) allows for high-contrast imaging at small inner working angles, however the performance is limited by the small throughput and the number of baselines. We present the concept and first lab results of Holographic Aperture Masking (HAM) with extreme liquid-crystal geometric phase patterns. We multiplex subapertures using holographic techniques to combine the same subaperture in multiple non-redundant PSFs in combination with a non-interferometric reference spot. This way arbitrary subaperture combinations and PSF configurations can be realized, giving HAM more uv-coverage, better throughput and improved calibration as compared to SAM, at the cost of detector space.

Hi-5 is a high-contrast (or high dynamic range) infrared imager project for the VLTI. Its main goal is to characterize young extra-solar planetary systems and exozodiacal dust around southern main-sequence stars. In this paper, we present an update of the project and key technology pathways to improve the contrast achieved by the VLTI. In particular, we discuss the possibility to use integrated optics, proven in the near-infrared, in the thermal near-infrared (L and M bands, 3-5 μm) and advanced fringe tracking strategies. We also address the strong exoplanet science case (young exoplanets, planet formation, and exozodiacal disks) offered by this wavelength regime as well as other possible science cases such as stellar physics (fundamental parameters and multiplicity) and extragalactic astrophysics (active galactic nuclei and fundamental constants). Synergies and scientific preparation for other potential future instruments such as the Planet Formation Imager are also briefly discussed.

With many thousands of exoplanets discovered one of the important next steps in astronomy is to be able to characterise them. This presents a great challenge and calls for new observational capabilities with both high angular resolution and extreme high contrast in order to efficiently separate the bright light of a host star to that of a faint companion. Glint South is an instrument that uses photonic technology to perform nulling interferometry. The light of a star is cancelled out by means of destructive interference in a photonic chip. One of the challenges is the star light injection into the chip. This is done by a unique active system that optimises the injection and provide low order correction for the atmospheric turbulence. We are reporting on the latest progress following several tests on the Anglo Australian Telescope.

A modern implementation of a stellar intensity interferometry (SII) system on an array of large optical telescopes would be a highly valuable complement to the current generation of optical amplitude interferometers. The SII technique allows for observations at short optical wavelengths (U/B/V bands) with potentially dense (u,v) plane coverage. We describe a complete SII system that is used to measure the spatial coherence of a laboratory source which exhibits signal to noise ratios comparable to actual stellar sources. A novel analysis method, based on the correlation measurements between orthogonal polarization states, was developed to remove unwanted effects of spurious correlations. Our system is currently being tested in night sky observations at the StarBase Observatory (Grantsville, Utah) and will soon be ported to the VERITAS (Amado, AZ) telescopes. The system can readily be integrated with current optical telescopes at minimal cost. The work here serves as a technological pathfinder for implementing SII on the future Cherenkov Telescope Array.

The Southern Connecticut Stellar Interferometer (SCSI) is a portable optical intensity interferometer located on the campus of Southern Connecticut State University in New Haven, Connecticut. Since its completion in 2016, the instrument has been used to take engineering data of bright stars. This paper will discuss the data collection and analysis methods, as well as the progress toward reliably measuring a significant stellar photon correlation. Vega has been the main star chosen for test observations to date because its diameter is well known by other methods, and it is not an extended source for the baselines used. The correlation peak in the processed data is compared to theoretical expectations. Given our expected sensitivity, a significant correlation peak is expected for small baselines (~2 m) to appear after a few hours of observation. So far, the observations indicate that the correlation peak is at the expected time delay, and the signal-to-noise ratio roughly scales as predicted.

The Southern Connecticut Stellar Interferometer (SCSI) is a two-telescope astronomical intensity interferometer that was completed in June 2016 and has been taking photon correlation data since that time. It uses single-photon avalanche diode (SPAD) detectors at the telescope focal plane and a central timing module, which records the signals from both telescopes simultaneously. In the observations taken to date, single-pixel SPADs have been connected to signal cables that stretch from each telescope to the timing module. However, we are now in the process of making the instrument “wireless” by using a separate timing module at each telescope and synchronizing the signals recorded using GPS timing cards. We have also upgraded one of the two stations with an 8-pixel SPAD device, which allows us to achieve higher count rates in a variety of observing conditions. In this paper, we report on the current state of the instrument, including engineering tests made in preparation for wireless operation, and we discuss the expected capabilities in that mode.

MATISSE is the second-generation mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This new interferometric instrument will allow significant advances in various fundamental research fields: studying the planet-forming region of disks around young stellar objects, understanding the surface structures and mass loss phenomena affecting evolved stars, and probing the environments of black holes in active galactic nuclei. As a first breakthrough, MATISSE will enlarge the spectral domain of current optical interferometers by offering the L and M bands in addition to the N band. This will open a wide wavelength domain, ranging from 2.8 to 13 μm, exploring angular scales as small as 3 mas (L band) / 10 mas (N band). As a second breakthrough, MATISSE will allow mid-infrared imaging - closure-phase aperture-synthesis imaging - with the four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. Moreover, MATISSE will offer a spectral resolution range from R ~ 30 to R ~ 5000. Here, we remind the concept, the instrumental design, and the main features of MATISSE. We also describe the last months of preparation, the status of the instrument, which was shipped to Cerro Paranal on the site of the ESO Very Large Telescope in October 2017, and the expected schedule for the opening to the community. The instrument is currently in its Commissioning phase. A complementary dedicated article details the Commissioning results, which include the first performance estimates on sky.

Recent work with the NESSI speckle camera at Kitt Peak and the 'Alopeke speckle camera at Gemini-North indicates that speckle data reduction techniques can be successfully modified to produce high-resolution images over fields that are at least tens of arc seconds across. While these wide-field speckle image reconstructions are not diffraction-limited, the improvement in resolution over the seeing-limited case can be substantial. In this paper, we explore the application of these techniques to data taken with a small (0.6-m) telescope in an urban environment. Many telescopes located in urban communities, such as New Haven, Connecticut, where Southern Connecticut State University resides, have limited use scientifically due to substantial light pollution, poor seeing, poor telescope tracking, and other issues. We will present initial data using our set-up and discuss the potential for this approach for improving the imaging capabilities of small telescopes on our campus and beyond.

Accurate fringe tracking is essential for sensitive long-wavelength thermal background limited operation of the current Very Large Telescope Interferometry (VLTI) and future Planet Formation Imager (PFI) facilities. We present and simulate a dual fringe tracking and low-order adaptive optics concept based on a combination of non-redundant aperture interferometry and eigenphase in asymmetric pupil wavefront sensing. This scheme can acquire fringes at many wavelengths of path length offset between telescopes, even with moderate tilt offset and pupil shifts between beams. Once locked to fringes, our technique can also be used for simultaneous low-order wavefront sensing, and has near-optimum sensitivity where there is a dominant point-source image component. This concept is part of the Heimdallr visitor instrument currently being investigated for VLTI.

Speckle imaging produces diffraction-limited images from ground-based telescopes. Recent advancements in detectors such as electron-multiplying CCDs (EMCCDs), have spawned a resurgence of this technique, greatly improving sensitivity and observing efficiency. The high angular resolution provided by speckle imaging can discern blended binary system contamination and validate suspected exoplanets discovered by the Kepler and K2 transit surveys. High-resolution follow-up will also be required for upcoming missions including TESS. Multiplicity can be determined along with separation, position angle, photometry, and contrast ratio. In this way, speckle imaging can validate even small, rocky planets like TRAPPIST-1 and constrain exoplanet radii and density. Some of the developments leading to this technique will be discussed in conjunction with recent significant papers, ongoing speckle imaging programs, and prospects for the future.

The Very Large Telescope Interferometer (VLTI) is a 4x8m + 4x1.8m telescopes array which has been operating as an optical interferometer for more than a decade now. We offer a prospective for the upcoming decade (2018-2028) taking into account the current state of VLTI, the science cases of the 2nd generation instruments (which started to be deployed in 2015), as well as the untapped possibilities offered by this unique facility.

The astronomical J-band (1.25 micrometres) is a relatively untapped wave-band in long-baseline infrared interferometry. It allows access to the photosphere in giant and super-giant stars relatively free from opacities of molecular bands. The J-band can potentially be used for imaging spots in the 1350 nm ionised iron line on slowly rotating magnetically-active stars through spectro-interferometry. In addition, the access to the 1080 nanometres He I line may probe out flows and funnel-flows in T-Tauri stars and allow the study of the star-disk interaction.

We present the progress in the development of a six-inputs, J-band interferometric beam combiner based on the discrete beam combiner (DBC) concept. DBCs are periodic arrays of evanescent coupled waveguides which can be used to retrieve simultaneously the complex visibility of every baseline from a multi-aperture interferometer. Existing, planned or future interferometric facilities combine or will combine six or more telescopes at the time, thus increasing the snapshot uv coverage from the interferometric measurements. A better uv coverage will consequently enhance the accuracy of the image reconstruction. DBCs are part of the wider project Integrated astrophotonics that aims to validates photonic technologies for utilisation in astronomy.

Before manufacturing the component we performed extensive numerical simulations with a coupled modes model of the DBC to identify the best input configuration and array length. The 41 waveguides were arranged on a zig-zag array that allows a simple optical setup for dispersing the light at the output of the waveguides.

The component we are currently developing is manufactured in borosilicate glass using the technique of multi-pass ultrafast laser inscription (ULI), using a mode-locked Yb:KYW laser at the wavelength of 1030 nm, pulse duration of 300 fs and repetition rate of 1 MHz. After annealing, the written components showed a propagation loss less than 0.3 dB/cm and a negligible birefringence at a wavelength of 1310 nm, which makes the components suitable for un-polarized light operation. A single mode fiber-to-component insertion loss of 0.9 dB was measured. Work is currently in progress to characterize the components in spectro-interferometric mode with white light covering the J-band spectrum.

The Planet Formation Imager (PFI) is a near- and mid-infrared interferometer project with the driving science goal of imaging directly the key stages of planet formation, including the young proto-planets themselves. Here, we will present an update on the work of the Science Working Group (SWG), including new simulations of dust structures during the assembly phase of planet formation and quantitative detection efficiencies for accreting and non-accreting young exoplanets as a function of mass and age. We use these results to motivate two reference PFI designs consisting of a) twelve 3m telescopes with a maximum baseline of 1.2km focused on young exoplanet imaging and b) twelve 8m telescopes optimized for a wider range of young exoplanets and protoplanetary disk imaging out to the 150K H2O ice line. Armed with 4 x 8m telescopes, the ESO/VLTI can already detect young exoplanets in principle and projects such as MATISSE, Hi-5 and Heimdallr are important PFI pathfinders to make this possible. We also discuss the state of technology development needed to make PFI more affordable, including progress towards new designs for inexpensive, small field-of-view, large aperture telescopes and prospects for Cubesat-based space interferometry.

The astronomical L band is particularly well suited for the hunt of low mass companions and the study of planet forming discs. In this paper, we present the concept of a spectro-interferometer with application to high precision interferometry, in projects such as Hi-5: a high-contrast thermal near-infrared imager for the VLTI. The interest of our system is that it allows, for the first time, spectro-interferometry in the mid-infrared (L-Band), in an integrated optic device, with a resolution of R=2000 in a 500μm long sampling zone. Fringe scan and photometry balancing are achieved on-chip, using an external applied voltage. This kind of devices has already been used for high contrast interferometry (36dB rejection ratio) and spectrometry, and first developments have been achieved in passive spectro-interferometers. This first demonstrator is a key milestone towards an interferometric nulling combiner dedicated to high contrast observations

This paper is one of a three-part series of papers on photonics-based mid-IR interferometry. Here, we put the emphasis on the challenges of operating integrated optics over a broad wavelength range, a natural condition in the field of Astrophysics. We report on the recent advancements made in obtaining high interferometric contrast (> 90%) through 2-telescope combiners in the mid-IR and give an outlook on more advanced functions and 4-telescope combiners.

Technology for image reconstruction in radio interferometry has evolved over nearly three decades with the state of the art moving towards fully automated data reduction and image reconstruction pipelines. There is potential merit in exploring areas of possible synergy with optical interferometry with the goal of transferring information both ways about lessons learnt and maybe even identifying areas where joint technical R and D may be mutually beneficial. This paper presents an overview of radio interferometric imaging and addresses the measurement of visibilities, the editing, calibration and image reconstruction steps. The state of the art is summarized and areas of potential overlap with optical interferometric data analysis and imaging are highlighted.

The limitations of adaptive optics and coronagraph performance make exoplanet detection close to λ/D extremely difficult with conventional imaging methods. The technique of non-redundant masking (NRM), which turns a filled aperture into an interferometric array, has pushed the planet detection parameter space to within λ/D. For high Strehl, the related filled-aperture kernel phase technique can achieve resolution comparable to NRM, without a dramatic decrease in throughput. We present non-redundant masking and kernel phase contrast curves generated for ground- and space-based instruments. We use both real and simulated observations to assess the performance of each technique, and discuss their capabilities for different exoplanet science goals such as broadband detection and spectral characterization.

The future of exoplanet detection lies in the mid-infrared (MIR). The MIR region contains the blackbody peak of both hot and habitable zone exoplanets, making the contrast between starlight and planet light less extreme. It is also the region where prominent chemical signatures indicative of life exist, such as ozone at 9.7 μm. At a wavelength of 4 μm the difference in emission between an Earth-like planet and a star like our own is 80 dB. However a jovian planet, at the same separation exhibits 60 dB of contrast, or only 20 dB if it is hot due to its formation energy or being close to its host star. A two dimensional nulling interferometer, made with chalcogenide glass, has been measured to produce a null of 20 dB depth, limited by scattered light. Measures to increase the null depth to the theoretical limit of 60 dB are discussed.

The new VLTI/GRAVITY instrument is a four telescope beam combiner installed at the VLT Interferometer. The principal novelty of this instrument is the availability of a dual field mode enabling narrow-angle relative astrometry at micro-arcsecond accuracy between two objects separated by several arcseconds. The fringe tracker (FT) stabilizes the interference fringes at up to 1 kHz frequency, allowing for long exposures with the science combiner (SC) as well as phase referenced imaging and differential astrometry (in dual field mode). The FT and SC beam combiners are integrated optics (IO) components, whose 24 outputs are (optionally) polarization-split and spectrally dispersed.
The processing of the photometric signals from the IO components is based on the pixel-to-visibility matrix (P2VM) formalism, that translates them into complex visibilities. The retrieval of the relative phase of the two objects subsequently relies on the combination of the phases measured from the FT, SC and the laser metrology. We will present the adopted algorithms, and an overview of the structure of the developed software. The calibration of the wavelength scales of the FT and SC at the required accuracy presents specific difficulties that we will briefly discuss. Examples of the reduction of on-sky data obtained during the commissioning will also be presented.

BETTII (Balloon Experimental Twin Telescope for Infra-red Interferometry) is designed to provide high angular resolution spectroscopic data in the far-infrared (FIR) wavelengths. The most significant limitation for BETTII is its sensitivity; obtaining spectral signal-to-noise ratio >5 in <10 minutes requires sources >13 Jy. One possible way to improve the signal-to-noise ratio (SNR) for future BETTII flights is by reducing the spectral bandwidth post beam-combination. This involves using a dispersive element to spread out a polychromatic point source PSF on the detector array, such that each pixel corresponds to a small fraction of the bandwidth. This results in a broader envelope of the interferometric fringe pattern allowing more fringes to be detected, and thereby improving the spectral SNR. Here we present the analysis and optical design of the dispersive backend, discussing the tradeoffs and how it can be combined with the existing design.

Proxima b is our nearest potentially rocky exoplanet and represents a formidable opportunity for exoplanet science and possibly astrobiology. With an angular separation of only 35 mas (or 0.05 AU) from its host star, Proxima b is however hardly observable with current imaging telescopes and future space-based coronagraphs. One way to separate the photons of the planet from those of its host star is to use an interferometer that can easily resolve such spatial scales. In addition, its proximity to Earth and its favorable contrast ratio compared with its host M dwarf (approximately 10-5 at 10 microns) makes it an ideal target for a space-based nulling interferometer with relatively small apertures. In this paper, we present the motivation for observing this planet in the mid-infrared (5-20 microns) and the corresponding technological challenges. Then, we describe the concept of a space-based infrared interferometer with relatively small (<1m in diameter) apertures that can measure key details of Proxima b, such as its size, temperature, climate structure, as well as the presence of important atmospheric molecules such as H2O, CO2, O3, and CH4. Finally, we illustrate the concept by showing realistic observations using synthetic spectra of Proxima b computed with coupled climate chemistry models.

One of the long-term goals of exoplanet science is the (atmospheric) characterization of a large sample (>100) of terrestrial planets to assess their potential habitability and overall diversity. Hence, it is crucial to quantitatively evaluate and compare the scientific return of various mission concepts. Here we discuss the exoplanet yield of a space-based mid-infrared (MIR) nulling interferometer. We use Monte-Carlo simulations, based on the observed planet population statistics from the Kepler mission, to quantify the number and properties of detectable exoplanets (incl. potentially habitable planets) and we compare the results to those for a large aperture optical/NIR space telescope. We investigate how changes in the underlying technical assumptions (sensitivity and spatial resolution) impact the results and discuss scientific aspects that influence the choice for the wavelength coverage and spectral resolution. Finally, we discuss the advantages of detecting exoplanets at MIR wavelengths, summarize the current status of some key technologies, and describe what is needed in terms of further technology development to pave the road for a space-based MIR nulling interferometer for exoplanet science.

The TOLIMAN space telescope is a low-cost, agile mission concept dedicated to astrometric detection of exoplanets in the near-solar environment, and particularly targeting the Alpha Cen system. Although successful discovery technologies are now populating exoplanetary catalogs into the thousands, contemporary astronomy is still poorly equipped to answer the basic question of whether there are any rocky planets orbiting any particular star system. Toliman will make a first study of stars within 10 PC of the sun by deploying an innovative optical and signal encoding architecture that leverages the most promising technology to deliver data on this critical stellar sample: high precision astrometric monitoring. Here we present results from the Foundational Mission Study, jointly funded by the Breakthrough Prize Foundation and the University of Sydney which has translated innovative underlying design principles into error budgets and potential spacecraft systems designs.

The Wide-field Imaging Interferometry Testbed (WIIT) is a double Fourier (DF) interferometer operating at optical wavelengths, and provides data that are highly representative of those from a space-based far-infrared interferometer like SPIRIT. We have used the testbed to observe both geometrically simple and astronomically representative test scenes. Here we present an overview of the astronomical importance of high angular resolution at the far infrared, followed by the description of the optical set-up of WIIT, including the source simulator CHIP (Calibrated Hyperspectral Image Projector). We describe our synthesis algorithms used in the reconstruction of the input test scenes via a simulation of the most recent measurements. The updated algorithms, which include instruments artifacts that allow the synthesis of DF experimental data, are presented and the most recent results analyzed.

In this contribution we review the recent advances in star and planet formation studies that have been allowed by long baseline optical interferometers in the last few years. We see how interferometry continues to bring significant observational constraints on complex processes such as accretion/ejection, dust sublimation and evolution and planet formation. We discuss the implication of the arrival on sky of GRAVITY and MATISSE at VLTI together with MYSTIC and MIRCX at CHARA and how these new facilities can contribute answering to the burning questions of planet formation.

PHASECam is the Large Binocular Telescope Interferometer's (LBTI) phase sensor, a near-infrared camera which is used to measure tip/tilt and phase variations between the two AO-corrected apertures of the Large Binocular Telescope (LBT). Tip/tilt and phase sensing are currently performed in the H (1.65 μm) and K (2.2 μm) bands at 1 kHz, and the K band phase telemetry is used to send tip/tilt and Optical Path Difference (OPD) corrections to the system. However, phase variations outside the range [-Π, Π] are not sensed, and thus are not fully corrected during closed-loop operation. PHASECam's phase unwrapping algorithm, which attempts to mitigate this issue, still occasionally fails in the case of fast, large phase variations. This can cause a fringe jump, in which case the unwrapped phase will be incorrect by a wavelength or more. This can currently be manually corrected by the observer, but this is inefficient. A more reliable and automated solution is desired, especially as the LBTI begins to commission further modes which require robust, active phase control, including controlled multi-axial (Fizeau) interferometry and dual-aperture non-redundant aperture masking interferometry. We present a multi-wavelength method of fringe jump capture and correction which involves direct comparison between the K band and currently unused H band phase telemetry.